Oxygen is separated from oxygen containing feeds, such as air, through cryogenic rectification. In cryogenic rectification, the feed is compressed, if not obtained in a pressurized state, purified of contaminants and then cooled in a main heat exchanger to a temperature suitable for its rectification. The cooled feed is then introduced into a distillation column system having high and low pressure columns in which nitrogen is separated from the oxygen to produce oxygen and nitrogen-rich product streams that warm within the main heat exchanger to help cool the incoming feed. As well known in the art, an argon column can also be provided that receives an argon-rich stream from the low pressure column and separates the argon from the oxygen to produce an argon containing product.
The oxygen that is separated from the feed can be taken as a liquid product that can be produced in the low pressure column as an oxygen-rich liquid column bottoms. Liquid product can additionally be taken from part of the nitrogen-rich liquid used in refluxing the columns. As known in the art, the oxygen liquid product can be pumped and then in part taken as a pressurized liquid product and also, heated in the main heat exchanger to produce an oxygen product as a vapor or as a supercritical fluid depending on the degree to which the oxygen is pressurized by the pumping. The liquid nitrogen can similarly be pumped and taken as either a pressurized liquid product, a high pressure vapor or a supercritical fluid. In order to heat the oxygen containing stream in the main heat exchanger, part of the feed can be further compressed, cooled and expanded into a liquid. The liquid can be introduced into either or both of the high and the low pressure columns.
In order to operate a cryogenic rectification plant, refrigeration must be supplied to offset ambient heat leakage, warm end heat exchange losses and to allow the production of liquid products. Refrigeration is typically supplied by expanding part of the air or a waste stream from the low pressure column within a turboexpander to generate a cold exhaust stream. The cold exhaust stream is then introduced into the distillation column or the main heat exchanger. External refrigeration can also be imparted by refrigerant streams introduced into the main heat exchanger. Refrigeration can also be generated through closed loop, external refrigeration cycles.
The main heat exchanger is typically formed by brazed aluminum, plate fin construction. In such a heat exchanger, layers containing fins, defined between parting sheets, form the passages for indirectly exchanging heat between the incoming streams and the return streams produced in the distillation columns. For example, layers are provided for indirectly exchanging heat between an oxygen-rich liquid stream that has been pumped and part of the feed stream that has been raised in pressure by a booster compressor. The main heat exchanger can be formed from several of such units and can be further separated into high pressure heat exchangers for heating the pumped oxygen-rich stream and low pressure heat exchangers for cooling the remainder of the incoming feed. In any event, the cost of such heat exchangers represents a major cost of the cryogenic rectification plant and typically, the price of a particular heat exchanger is based upon its volume.
Where air is expanded for providing the refrigeration, part of the air, after having been compressed and purified is further compressed in a booster compressor, partially cooled within the main heat exchanger and then is expanded in a turboexpander coupled to the booster compressor. This arrangement is known in the art as a turbine loaded booster compressor. In any case, since the air is partially warmed to a temperature between the warm and cold end temperatures of the main heat exchanger, portions of layers remain open for use in other heat exchange duties. In a pumped liquid oxygen plant, these portions can be used in cooling part of the air or feed stream that is provided for warming the pumped liquid oxygen. This of course reduces the size and cost of the main heat exchanger that would otherwise exist if these portions of the layers were left unused.
As will be discussed, the present invention provides a method of producing an oxygen product by cryogenic rectification or an apparatus for conducting such cryogenic rectification with the object of producing high pressure oxygen in which the main heat exchanger is able to be fabricated in either a more compact manner than that contemplated in the prior art or alternatively, for a given size of heat exchanger, higher volumetric flows are able to be brought into an indirect heat exchange relationship. Moreover, such a heat exchanger can be integrated to accept an external refrigerant stream to increase production of liquid products if the same are produced by the plant.